Recovery of aromatics



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INVENTOR:

ELDRED E. YOUNG BYIW- HIS ATTORNEY United States Patent 20 3,209,047 RECOVERY OF AROMATI'CS Eldred E. Young, Concord, Califi, assignor to Shell Oil Company, New York, N .Y., a corporation of Delaware Filed Sept. 21, 1961, Ser. No. 139,778 6 Claims. (Cl. 260674) This invention relates to an improved extraction process for recovering aromatic components from aromatic-containing feedstocks. More specifically, this invention is directed to an improved extraction process for the recovery of high-purity aromatics wherein the solvent passing from the extractor is recovered and recycled to the extractor without contaminating the extract removed from the extractor with non-aromatics, thereby producing a highpurity aromatic extract.

A conventional process employed for the recovery of high-purity aromatics from various feedstocks is the extraction of such mixtures with selective solvents such as sulfolane or diethylene glycol. FIGURE 1 is a schematic diagram of such a conventional process. In a process of this type, it is generally experienced that some solvent dissolves in the rafiinate passing overhead via line 42 from extractor 43. This solvent can be conveniently recovered by washing the raflinate with water in a raflinate washer 44. The resultant water-solvent mixture can then be introduced directly (not shown) into solvent stripper 46, where the solvent is recovered and the water serves as stripping steam. However, where high-aromatic purities are desired a difiiculty arises in that any saturated hydrocarbons dissolved or entrained in the rafiinate wash water would subsequently be mixed with the extract phase containing aromatics which is withdrawn from extractor 43, thereby contaminating the aromatic product. Because of this difiiculty, it has been common practice heretofore to provide a water still 41 as shown in FIGURE 1 of the drawing to recover the lean solvent from the wash water and return it to the extractor via line 47 while distilling the water and the entrained raflinate overhead via line 48, thereby avoiding contaminating the extract.

It is an object of this invention to provide an improved extraction process for the separation of high-purity aromatics from aromatic-containing feedstocks. It is a further object of this invention to provide an improved extraction process wherein the solvent passing overhead from the extractor with the raffinate is recovered without contaminating the extract phase with components of the raflinate. It is a further object of this invention to provide an improved extraction process for the recovery of high-purity aromatics wherein rafiinate wash water is used as stripping water in an extractive stripper, thereby avoiding the use of a water still for solvent recovery. Another object of the invention is to provide an extraction process wherein the selectivity of the selective solvent is enhanced and the solvent circulation rate is reduced. A further object of the invention is toprovide an improved extraction process wherein solvent loss due to decomposition is reduced. Other objects and advantages of this invention will become apparent in the description thereof which is made with reference to the accompanying drawing which consists of FIGURES 1 and 2. FIGURE 1, as discussed above, is a schematic diagram of a conventional extraction process wherein a water still is used to recover a solvent phase and a rafiinate/ water phase from the rafiinate wash water, thus avoiding contaminating the aromatic-rich extract with raffinate. FIGURE 2 is a schematic diagram of a preferred embodiment of the invention wherein raffinate wash water is used as stripping water in an extractive stripper, thereby avoiding the use of a water still for solvent recovery while producing a high-purity aromatic product.

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Accordingly, the invention is directed to an improved extraction process for recovering aromatic components from a mixed hydrocarbon feedstock wherein the hydrocarbon feedstock is introduced into a solvent extraction zone and the aromatic contained therein are selectively dissolved by .a selective solvent such as sulfolane. A small portion of the solvent is dissolved in the raffinate phase and passes overhead with the rafiinate from the extractor. This portion of the solvent is subsequently recovered by washing the rafiinate with water. It has now been discovered that the rafiinate wash water can be used as stripping water in an extractive stripper (a column in which the more volatile components contained in the fat solvent are partially distilled overhead) without danger of contaminating the aromatic extract introduced into the stripper with the raffinate components contained in the rafiinate wash water. For it has now been discovered that by introducing the rafiinate wash water into an extractive stripper at a point preferably above the bottom tray, any saturated hydrocarbons entrained in the solvent are vaporized and returned to the extractor along with the tops from the extractive stripper stream with the high-purity aromatics being Withdrawn as extractive stripper bottoms. The water, being relatively non-volatile as compared to the saturated hydrocarbon raffinate, will be largely transferred with the extractive stripper bottoms to a solvent stripper where it can be vaporized as stripping steam.

Introduction of the ratfin-ate wash water (or other water not used for raifinate washing) into the lower part of the extractive stripper has the following advantages:

(1) The relative volatility of the saturates compared with aromatics is greater in the presence of water than in the presence of solvents such as sulfolane or diethylene glycol. Therefore, the addition of water to the solvent enhances the selective stripping of saturates, thereby reducing significantly the aromaticity of the raflinate passing overhead from the extractive stripper. Therefore, by adding water to the extractive stripper a reduction in the aromaticity of the overhead recycle stream is elfected while recovery of high-purity aromatics is accomplished. For, example, it has been determined that the relative volatility of normal nonane with respect to benzene would be about 5 in a conventional operation such as shown in FIGURE 1, with 5% mole water in sulfolane in the extractive stripper bottoms, whereas in the process as disclosed in FIGURE 2 with 20% mole water in sulfolane, the relative volatility of normal nonane with respect to benzene is estimated to be about 10. Further, the addition of water serves to reduce the required solvent circulation rate, resulting in savings both in capital investment and operating cost.

(2) A lowering of the extractive stripper reboiler temperature is effected, thereby reducing the solvent decomposition rate caused by high-temperature operation. Reducing reboiler temperature also makes it possible to improve heat economy, for example, by allowing the extractive stripper reboil heat to be supplied by exchange with stripper bottoms, as shown in the dotted lines in FIGURE 2.

In the extractive process of the invention, the conditions of temperature, pressure and concentration of solvent in the extraction zone are selected to insure optimum recovery of high-purity aromatics from the feed charge. A lean, relatively aromatic-free solvent selective for aromatic hydrocarbon-s and which is relatively immiscible with non-aromatic hydrocarbons is admitted to one end of an extraction zone and in extractive contact with the aromatics contained in the feed charge. Such a solvent should have a relatively high selective solubility for aro-.

matics at the extraction temperature. The liquid feed is introduced at a point midway up the extraction column. The solvent flows through the zone countercurrent to the feed charge. The fat solvent containing the selectively dissolved aromatics is continuously withdrawn from the ex- 7 traction Zone at the end opposite the end where the feed is introduced, and is then introduced into an extractive stripper. The raffinate containingsome of the solvent passes out of the extraction zone and is introduced into a raffinate washer to remove the solvent. The solventcontaining wash water from the raftinate Washer is then introduced into the lower section of an extractive stripper. The distillate from the extractive stripper containing the non-aromatic hydrocarbons is recycled to the extractor as part of the feedstock, with the water-sulfolane-aromatic hydrocarbon extractive stripper bottoms being introduced into a solvent stripper. The stripped lean solvent is then recycled to the extractor with the aromatics product passing overhead along with the water. Subsequently the aromatics are separated from the water as a high-purity aromatic product.

The selective solvents which can be used according to the invention should be selective to aromatics, and must have some miscibility with water to insure that the solvent will pass out of the bottom of the extractive stripper along with the water. Further, the solvent must be stable at the extractive stripper and solvent stripper temperatures. That is, the solvent must be sufficiently heat-stable so that it may at least partially vaporize without decomposition in the presence of the components of the mixture to be selectively extracted. Among the preferred solvents are the glycols, including particularly diethylene glycol, and the various sulfolane solvents. Various other common selective solvents for aromatics such as phenol, nitrobenzene, acetonitrile and furfural may also be used.

In a preferred embodiment of the invention, sulfolane solvents having the following general formula are used in the improved extraction process:

H H R B E R H- H R s R \O where R can be a hydrogen atom, an alkyl group having up to 10 carbon atoms, an alkoxy radical having up to 8 carbon atoms, an aryl alkyl radical having up to 12 carbon atoms or mixtures thereof. In a particularly preferred embodiment, all Rs are hydrogen.

Many different aromatic/non-aromatic mixtures may be separated by the extraction process of the present invention. However, a preferred embodiment of the invention is the recovery of aromatics from hydrocarbon feed mixtures containing substantial concentrations of aromatics.

The apparatus employed in this process may be any conventional or convenient type known to those skilled in the art. For simplicity the drawings do not show all the pumps, tanks, heat exchangers, valves, bypasses, vents, reboilers, condensers, coolers and other auxiliaries that may be necessary for the proper operation of the process, but the inclusion of which will be evident to those skilled in the art.

The temperature for the extraction column may range within wide limits provided it is below the bubble temperature of the mixture and above the solidification temperature of the selective solvent under the pressure and temperature conditions maintained in the column. The most suitable operating temperatures vary with the pressure maintained during the operation and are generally a compromise since in general the selectivity of these solvents seems to decrease with an increase in temperature. In a preferred embodiment of the invention, substantially atmospheric pressures are maintained in the extraction column.

The amount of solvent should be at least suificient to dissolve the constituent to be extracted. In some cases a considerable excess over this amount may be used, especially when it is desired to recover the last traces of the more soluble constituents from the raffinate. The most economic amount varies with the nature of the particular selective solvent being used and the temperatures and pressures employed in the particular feed mixture. This amount can be determined experimentally. Useful solvent to feed ratios in the extraction zone may range from about /2:1 to about 20:1 by volume and preferably not more than about 5:1 by volume.

The optimum temperature to be employed in the extraction step varies with the particular feedstock to be treated and the solvent to be used, and may range up to about 350 F. or more, but in most instances temperatures below about 250 F. and particularly those in the range of from about to about 200 F. are preferred.

Referring now to FIGURE 2 of the drawing, the feed stream comprising an aromatic-containing feedstock is introduced via line 1 into extractor 2. Extractor 2 can be a conventional liquid-liquid extraction column, such as a column containing a plurality of perforated plates horizontally positioned therein, or in a preferred embodiment of the invention, column 2 is a rotating disc contactor, as described in US. Patent to Reman, 2,601,674 issued June 24, 1952.

A lean solvent stream containing water is introduced via line 3 into the upper portion of extractor 2. The selective solvent flows down column 2 countercurrent to the feed charge and selectively dissolves the aromatics therefrom with the fat solvent containing the selectively dissolved aromatics and water passing out of extractor 2 via line 4. The fat solvent is then introduced into the top of extractive stripper 5.

The raflinate from extractor 2 is passed overhead via line 6 to raffinate washer 7. The raffinate normally contains in addition to the non-aromatic hydrocarbons a portion of the selective solvent. The ratfinate washer can be any conventional contacting device and in a preferred embodiment of the invention, it is a rotating disc contactor. Wash water is introduced into raffinate wash column 7 via line 9. Washed raffinate (solvent-free) is passed out of the raflinate washer 7 via line 8. The ratfinate wash water containing the solvent and some entrained hydrocarbons is passed out of column 7 via line 10 and is introduced into extractive stripper 5 at a point near the bottom thereof. In a preferred embodiment the ralfinate wash water is introduced into extractive stripper 5 at a point above the bottom tray such that any saturated hydrocarbons contained therein will be vaporized and returned to feed line 1 by means of recycle stream 11. Therefore, any saturated hydrocarbons introduced into extractive stripper 5 with the rafiinate wash water via line 10 or with the fat solvent via line 4 is passed out of the extractive stripper as overhead via line 11 and recycled and reintroduced into feed charge line 1, thereby improving the purity of the aromatic product withdrawn from the extractive stripper via line 13. Extractive stripper column 5 is equipped with reboiler 12.

The extract phase taken from the bottom of extractive stripper 5 via line 13 contains the extracted aromatics in addition to the selective solvent and water. This mixture is introduced into solvent stripper 14 at a point midway up to stripper column. The stripped aromatics are passed overhead along with the water. The water is separated from the aromatic product (not shown) and returned to rafiinate washer 7 via line 9 with the water-free high-purity aromatics passing out of the system as an end product. The lean selective solvent, which may contain some water, is introduced into extractor 2 via line 3 and can be passed through reboiler 12 prior to introduction into extractor 2 (see dotted lines), thus serving as a heat-exchange medium. Solvent stripper 14 is equipped with reboiler 15.

The lean solvent can contain up to about 20% by volume water, with the exact concentration desired being maintained by controlling solvent stripper column conditions. Controlling the stripper column conditions will not endanger the recovery of a high-purity aromatic overhead since the control of the stripper column is merely a function of steam rate and afiects only the material coming into the stripper. Valved line 16 is provided for the addition of fresh solvent to the system as required. A slip stream of lean solvent of approximately to by volume of the total solvent inventory can be taken by line 17 to a solvent clean-up flasher or vacuum column (not shown) during each 24-hour period to remove contaminants. After cleanup, the solvent can be returned to the system through line 3.

I claim as my invention:

1. A liquid-liquid solvent extraction process for separating an aromatic product from a mixture of aromatic and non-aromatic which comprises:

(1) contacting said mixture in an extraction zone with a water-miscible, aromatic-selective solvent boiling above the boiling point of the aromatics to be extracted,

(2) recovering a raflinate phase comprising non-aromatics and a minor amount of solvent, and an extract phase comprising aromatics, solvent and a minor amount of nonarornatics,

(3) water-washing said raflinate to recover solvent therefrom as a water wash mixture of solvent, water, and a minor amount of non-aromatics,

(4) introducing the extract phase into the top of an extractive stripping zone wherein distilling takes place for recovery of a first distillate comprising non-aromatics for return to the extraction zone and a first bottoms product comprising solvent, water, and aromatics,

(5) introducing said water-wash mixture of solvent, water, and minor amount of non-aromatics into the lower portion of the extractive stripping zone whereby non-aromatics of the water-wash mixture are recovered in the first distillate and returned to the extraction zone,

(6) distilling said first bottoms product in a solvent stripping zone for recovery of a second distillate product comprising said aromatic product and a second bottoms product comprising lean solvent and water for return to the extraction zone.

2. A process according to claim 1 wherein the extraction zone is equipped with a rotating disc contactor.

3. A process according to claim 1 wherein the selective solvents contain from about 1 to about 20% by volume water.

4. A process according to claim 1 wherein the selective solvent is selected from the group consisting of sulfolane solvents, glycol solvents, phenol solvents and furfural.

5. A process according to claim 4 wherein the selective solvent is sulfolane.

6. A process according to claim 4 wherein the selective solvent is diethylene glycol.

References Cited by the Examiner UNITED STATES PATENTS 2,773,918 12/56 Stephans 260674 2,792,332 5/57 Hutchings 260674 2,886,610 5/59 Georgian 260-674 2,921,015 1/ Shiras 208-321 2,936,283 5/ 60 Hutchings 208-321 ALPHONSO D. SULLIVAN, Primary Examiner. 

1. A LIQUID-LIQUID SOLVENT EXTRACTION PROCESS FOR SEPARATING AN AROMATIC PRODUCT FROM A MIXTURE OF AROMATIC AND NON-AROMATIC WHICH COMPRISES: (1) CONTACTING SAID MIXTURE IN AN EXTRACTION ZONE WITH A WATER-MISCIBLE, AROMATIC-SELECTIVE SOLVENT BOILING ABOVE THE BOILING POINT OF THE AROMATICS TO BE EXTRACTED, (2) RECOVERING A RAFFINATE PHASE COMPRISING NON-AROMATICS AND A MONOR AMOUNT OF SOLVENT, AND AN EXTRACT PHASE COMPRISING AROMATICS, SOLVENT AND A MINOR AMOUNT OF NON-AROMATICS, (3) WATER-WASHING SAID RAFFINATE TO RECOVER SOLVENT THEREFROM AS A WATER WASH MIXTURE OF SOLVENT, WATER, AND A MINOR AMOUNT OF NON-AROMATICS, (4) INTRODUCING THE EXTRACT PHASE INTO THE TOP OF AN EXTRACTIVE STRIPPING ZONE WHEREIN DISTILLING TAKES PLACE FOR RECOVERY OF A FIRST DISTILLATE COMPRISING NON-AROMATICS FOR RETURN TO THE EXTRACTION ZONE AND A FIRST BOTTOMS PRODUCT COMPRISING SOLVENT, WATER, AND AROMATICS, (5) INTRODUCING SAID WATER-WASH MIXTURE OF SOLVENT, WATER, AND MINOR AMOUNT OF NON-AROMATICS INTO THE LOWER PORTION OF THE EXTRACTIVE STRIPPING ZONE WHEREBY NON-AROMATICS OF THE WATER-WASH MIXTURE ARE RECOVERED IN THE FIRST DISTILLATE AND RETURNED TO THE EXTRACTION ZONE, (6) DISTILLING SAID FIRST BOTTOMS PRODUCT IN A SOLVENT STRIPPING ZONE FOR RECOVERY OF A SECOND DISTILLATE PRODUCT COMPRISING SAID AROMATIC PRODUCT AND A SECOND BOTTOMS PRODUCT COMPRISING LEAN SOLVENT AND WATER FOR RETURN TO THE EXTRACTION ZONE. 